Avalanche Forecast

Archived

Dec 1st, 2017–Dec 2nd, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Northwest Inland.

The late October crust near the base of the snowpack is reactive to rider triggers producing large avalanches. Its possible to trigger this from a significant distance away. Heads up out there as this problem will likely persist.

Confidence

Low - Due to the number and quality of field observations

Weather Forecast

Saturday: Mostly cloudy with isolated flurries and alpine temperatures near -15. Ridgetop winds light from the North West and freezing levels at valley bottom.Sunday: Mostly cloudy in the valley with sunshine at higher elevations. Alpine temperatures near -12and freezing levels at valley bottom. Ridgetop winds light from the West.Monday: Cloudy with new snow amounts 5-10 cm. Ridgetop winds moderate from the South West. Alpine temperatures rising to 0 degrees and freezing levels near 1100 m.

Avalanche Summary

No recent avalanche activity has been reported. I suspect stiff wind slabs exist at treeline and in the alpine on leeward slopes and behind terrain features. If triggered, these wind slabs could step down and trigger deeper weak layers within the snowpack initiating large avalanches. If you get out this weekend please submit your observations to the Mountain Information Network.

Snowpack Summary

Alpine snow surfaces are likely wind affected with the average snowpack depths 80 cm -140 cm at treeline elevations, and up to 160 cm in the alpine. Approximately 30-50 cm of recent snow now overlies two crusts that were buried near the end of November. Reports indicate that these crusts extend well into the alpine and seem to be supportive to rider traffic. Below these crusts is a well settled mid pack that sits on the late October crust close to the base of the snowpack at all elevations. This late October "crust/facet combo" is widespread and has been very reactive to rider triggers producing large avalanches.

Problems

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.